A hot rolled steel sheet (hot rolled steel strip or steel plate) is manufactured by rolling a slab heated at a high temperature such that the slab has a desired size. In this case, the hot rolled steel sheet is cooled using cooling water by a cooling apparatus during hot rolling or after finish rolling. As for the purpose of water cooling (cooling with cooling water), controlling deposit or transformed structure of the hot rolled steel sheet by water cooling mainly adjusts the quality of material in order to obtain intended strength, ductility, and the like. In particular, accurately controlling a finish cooling temperature is the most important to manufacture hot rolled steel sheets having intended material properties without variation.
As a result of soaring rare metal costs, methods have been developed which improve mechanical properties by transformed structure control based on cooling instead of alloy composition adjustment. In the above-described water cooling, there is a great need for wide range control of cooling rate in response to requirements for material quality.
In a run-out table of a typical cooling apparatus in a hot rolled steel strip manufacturing line, arrangement of pipe laminar cooling for the upper surface and spray cooling for the lower surface is often used. The flow rate of cooling water is about 700 to 1000 L/min·m2. For a steel strip having a thickness of 3 mm, a cooling rate of about 70° C./s is provided. For 25 mm steel which has a typical thickness of a relatively thick steel strip (high tensile strength steel for shipbuilding or steel for line pipe), a cooling rate of about 10° C./s is provided in this cooling apparatus.
In the hot rolled steel strip manufacturing line, steel strips to be processed have a wide range of thicknesses, 1.2 to 25 mm. In addition, for example, steel emphasizing workability and steel emphasizing toughness are processed. There is a need to increase a cooling rate only when a thick steel strip is processed. As a method of regulating the cooling rate, it is often necessary to regulate the cooling water flow rate.
In the hot rolled steel strip manufacturing line, the passing performance of a steel strip varies depending on, in particular, thickness. Unfortunately, difficulties occur. Specifically, for example, as regards high tensile steel for automobile, most of steel strips have thicknesses from about 1.2 to 3.0 mm. Such a steel strip of this size has poor stiffness and provides high passing speed. Accordingly, during conveyance by table rollers, a lift force caused by air resistance or fluid resistance generated by cooling water is applied to the steel strip, so that the steel strip tends to bounce. In particular, an ultrathin steel strip having a thickness of about 1.2 mm bounces up to about 1000 mm on a pass line. It therefore may be necessary to cool a thin steel strip on the pass line from a distance of 1000 mm or more at a relatively low water flow rate. Accordingly, a related-art run-out table uses a pipe laminar cooling unit capable of performing distant cooling to cool the upper surface of a steel strip.
In the typical cooling apparatus with the arrangement of pipe laminar cooling for the upper surface and spray cooling for the lower surface, however, cooling at a high water flow rate has various problems.
For example, as the cooling water flow rate of upper-surface pipe laminar cooling is increased, the velocity of flow in each pipe is extremely increased. Thus, the spray of cooling water changes from continuous laminar flow to jet flow. In the pipe laminar, cooling water is sprayed from pipes, each having a nozzle orifice diameter of about 10 to 25 mm, arranged at a distance of about 1000 to 1500 mm from the steel strip conveying line. Disadvantageously, part of jetted cooling water is formed into droplets, so that the continuity of cooling water is lost. In addition, since part of the jetted cooling water spatters, efficient cooling is not achieved.
It is therefore difficult to remarkably change a cooling rate during cooling of a hot rolled steel strip on the run-out table. Traditionally, the composition of steel has been mainly adjusted so as to be suitable for established cooling rates.
As regards steel plates, the thicknesses of products range from 6 to 100 mm, namely, the variations in thicknesses are remarkably wide. As a steel plate is thicker, the cooling rate decreases. Accordingly, as a steel plate is thicker, the amounts of alloying elements are increased to satisfy mechanical properties, such as strength and toughness. There is therefore a need to increase a cooling rate for a thicker steel plate as much as possible in order to reduce a change in cooling rate depending on thickness, as in the case of hot rolled steel strips.
To solve the above-described problem, for example, as means for ensuring a cooling rate for thick-sized steel plate, a method of cooling with a group of columnar jet flows is disclosed in PTL 1 and PTL 2, which describe a technique of spraying cooling water from a position relatively close to a steel sheet to achieve uniform cooling.
PTL 3 discloses a technique of spraying cooling water through slit nozzle units, provided with an elevating mechanism, arranged so as to face each other in a conveying direction and ensuring a cooling rate in a wide range using laminar nozzles and spray nozzles separated from the units such that stable cooling is achieved.